Well, the genre is kinda big, and divided into lots of sub categories, all with a very distinct sound.

My personal contemporary favorites include
MF Doom (check out Operation: Doomsday)
Madlib (everything, but most notably Quasimoto: the Unseen (very unconentional album) and the album Madlib and Doom did together: Madvillian)
Jay-dee (just lot's of little songs everywhere. Check out The Red by Jaylib)
El-p (check out Fantastic Damage and his old group Company Flow's album Funcrusher Plus, both very difficult listens for many)
Cannibal Ox - The Cold Vein
MF Grimm (the downfall of Iblyss: a ghetto operah).

If you like geeky white canadian angst-rap, chek out Buck 65 - Vertex.

Classic albums to check out are
Nas - (before he went pop) Illmatic (greatest hiphop album of all time)
Gang Starr - Step in the Arena
The Roots - Do you want more??!?!?!
Everything Wu-tang Clan did between 93 and 96 (my favorites are Raekwon - Only Built for Cuban Linx and Wu-tang Clan - Enter the 36 Chambers),
Pete Rock and CL Smooth - Mecca and the Soul Brother (jazzy ****) and The Main Ingredient (a bit more mellow and accessible)
KMD - Mr Hood.
A lot of people would say Public Enemy, but I never got into them.
Run DMC are also good, but they're not my favorite group